Pump affinity law

This pump had about the same performance as the electric motor driven unit. At variable rpm it followed the pump affinity laws quite closely, and so will not be further discussed. 

Pump affinity law is used to estimate pump performance, when its speed or impeller diameter is chmiged. It is derived based on pump specific speed doesn t change. The first set of pump affinity law is based on fixed impeller diameter and varied pump speed. It is used to estimate pump performance at different pump speed. The new pumping rale (Q2), pump head (H2), and required power (BHP2) are estimated from the known pumping rate (Ql), pump head (HI), and power (BHPl) by the following equations ... 

In general, the first set of pump affinity law is more accurate than the second set. The second set of pump affinity law is less accurate, if pump diameter is reduced too much or pump has higher specific speed. 

Affinity Laws. Pump performance is affected by the rotating speed. When speed increases, the flow increases linearly, and the head increases as a square of the speed (14). 

If detailed manufacturer-specified performance curves are not available for a different size of the pump or operating condition, a best estimate of the off-design performance of pumps can be obtained through similarity relationship or the affinity laws. These are ... 

When a pump s velocity changes, measured in revolutions per minute (rpm), the operational characteristics aLso change. These changes can be calculated using the Affinity Laws. Before continuing, let s define some-terms we ll be using ... 

Manipulating flow and controlling prcs.surc by varying the impeller diameter conserve.s kilowatts of energy, and this is the third affinity law in this group. A pump consuming 10 BHP with a 10 inch impeller, would only consume 7.3 horses with a 9 inch impeller. 

Actually, everything we said about bearings, mechanical seals, piping, TDH, system curves and mating the pump curve to the system curve, the affinity laws, cavitation, horsepower and efficiency arc as applicable to PD pumps as centrifugal pumps. 

Small impellers with high motor speeds may produce the necessary pump pressure. This ty pe of combination produces high fluid velocities that will wear pump parts much faster than desirable. This is in the Affinity Laws. In addition the impeller suffers rapid wear due to high tip velocities. When a pump is disassembled and excessive wear is found, 95% of the time high velocity fluid is to blame. 

Dynamic type pumps obey the affinity laws ... 

The affinity laws relate the performance of a known pump along its characteristic curve to a new performance curve when the speed is changed. This would represent the same family of pump curves. As an example, see Figures 3-36A, B, and G. 

Recognizing the flexibility of the affinity laws, it is better to select an original pump impeller diameter that is somewhat larger than required for the range of anticipated performance, and then cut this diameter dowm after in-service tests to a slightly smaller diameter. This new performance can be predicted in advance. Once the impeller diameter is too small, it cannot be enlarged. The only solution is to order the required large impeller from the manufacturer. 

Pumps, centrifugal system performance, 197 Affinity laws, 201-203 Branch piping, 200 Calculations, 199... 

Centrifugal potting, 16 18 Centrifugal pumps, 19 513 affinity laws related to, 21 63 costs associated with, 21 87 efficiency of, 21 60 nonmetallic, 21 76 suction specific speed of, 21 63 Centrifugal sedimentation, 18 142, 143-144 Centrifugal separation(s), 5 505-551 ... 

Two different size pumps are said to be geometrically similar when the ratios of corresponding dimensions in one pump are equal to those of the other pump [Holland and Chapman (1966)]. Geometrically similar pumps are said to be homologous. A set of equations known as the affinity laws govern the performance of homologous centrifugal pumps at various impeller speeds. 

Equations 4.24 to 4.27 are the affinity laws for homologous centrifugal pumps. 

SIMILARITY OR AFFINITY LAWS IN CENTRIFUGAL-PUMP SELECTION 6.31... 

Compute the pump performance at the new speed. The similarity or affinity laws can be stated in general terms, with subscripts p and m for prototype and model, respectively, as Qp = K KnQm-,Hp = K2dK2Hm- NPSH = K2K2NPSHm Pp = K3K3Pm, where Kd = size factor = prototype dimension/model dimension. The usual dimension used for the size factor is the impeller diameter. Both dimensions should be in the same units of measure. Also, Kn = prototype speed, r/min/model speed, r/min. Other symbols are the same as in the previous example. 

Horowitz— Affinity Laws and Specific Speed Can Simplify Centrifugal Pump Selection, Power, November 1964. 

The performance curves published by manufacturers of chemical process pumps are usually based on data obtained on pumps that have a single, multispring, mechanical seal in the stuffing box (Fig. 1). The curves are usually shown for 3,550 and 1,750 rpm. Many pump manufacturers run their tests at the design speed, 3,550 rpm, and use the affinity laws (summarized later) to calculate performance at 1,750 rpm. 

Fig. 5 shows operating horsepower vs. additional decrease in points of pump efficiency when using a single or double seal, based on performance data that have been converted from 3,550 rpm to 1,750 rpm by use of the affinity laws. 

The increase in horsepower required should be applied to the operating pump horsepower to calculate the resulting efficiency. This differential is greater when pump horsepower has been stepped from one speed to another by using the affinity laws (rather than by actually measuring efficiency at the speed in question). 

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